No. 2. J COPE ON MIOCENE RODENTIA. 375 



series. The inferior incisor is rather hirge. Its section is nearly trian- 

 gular. The enamel face is entirely smooth. 



Measurements. M. 



Length of ramus iucluded. in chord of incisor tooth 0300 



Length of inferior nioltir series 0200 



Diameters grinding surface M. I)^ ^ ^^^^, 



^ transverse OOaO 



Elevation grinding surface, M. I. , inner side 00:!2 



Length of diastema 0100 



Width anterior surface of incisor 0050 



Depth of ramus at M. Ill 0155 



This species was about the size of the wood-chuck {Arctomys monax), 

 to judge by the dimensions of its lower jaw. It is larger than the ill. 

 sesquipedalis, and has a different arrangement of the enamel fossettes. 

 In that species, instead of being in three parallel lines, the middle line 

 is only represented by its extremities. At the middle of the crown the 

 fossettes of the internal line are incurved so as to be nearly in contact 

 with the fossettes of the external line. 



The Mylagaulus monodon was discovered by Wilbur J. Brous in Hitch- 

 cock County, Nebraska. 



HELISCOMYS Cope. 



Synopsis of New Vertebrata from Colorado (Miss. Pub. U. S. Geol. Survey Terrs. ), 1873 

 (October), p. 3; Annual Eeport U. S. Geol. Survey Terrs. 1873 (1874), p. 475. 



Inferior molars four, rooted, the crowns supporting four cusps in trans- 

 verse ijairs. A broad ledge or ciugulum projecting on the external side 

 from base of the cusps. The inferior incisor compressed, not grooved, 

 and with the enamel without sculj3ture. 



This genus is only represented by a small number of specimens, 

 which are mandibular rami exclusively. Its special afduities therefore 

 cannot be ascertained, and even its general position remains somewhat 

 doubtful. There is some probability however that it belongs to the 

 Myomorpha,, as the tj-pe of dentitiou is much more like that of the gen- 

 era of that group than those of the Sciuromorplia. To the Hystrlco- 

 morpha it does not belong. 



As comisared with known genera of Myomorpha., it is at once separ- 

 ated from many of them by the presence of a premolar tooth. Among 

 recent genera of this suborder, Sminthus possesses this tooth in both 

 jaws, and Meriones in the upper jaw only. It is present in both jaws in 

 the Sciuromorpha generally. The tubercles of the teeth resemble those 

 of the 2Inrid(C, but their disposition is unlike that of any existing Xorth 

 American genus. A remote ai^proximation to it is seen in the genus 

 Syllophodns* of the Bridger Eocene formation, where there are four 

 subquadrate molars with tubercles; but the latter form two transverse 

 crests, with an additional small intermediate tubercle, and the wide 

 cingulum is absent. 



But one species of Heliscomys is known, the H. i-etus. 



* Myops Leidy (^" Mysops") i)reoccupied. 



